What is the best approach to build a self-learning AI chatbot?
The former: build and train a model first, and then think about the user interface.

Effectively, a chatbot is a user interface to your model. If you run it 'off-line' on input text and it works, then you have achieved your goal without the added complexity of driving a conversation (which is harder than one might think).

Also, building an 'abstract' chatbot devoid of any content is going to be harder. What should that chatbot talk about?

From my own experience (I work in conversational AI), you might not succeed in building a decent chatbot, especially as a novice. But you might be able to train a model to identify diseases from textual input. So if you do that first, you have at least got something! Especially if that is your main reason for the project in the first place.